Subject: [PATCH] arm64: add prctl(PR_PAC_SET_ENABLED_KEYS)

From: Peter Collingbourne <[email-masked]>
Date: Fri, 31 Jul 2020 18:11:52 -0700

This prctl allows the user program to control which PAC keys are enabled
in a particular task. The main reason why this is useful is to enable a
userspace ABI that uses PAC to sign and authenticate function pointers
and other pointers exposed outside of the function, while still allowing
binaries conforming to the ABI to interoperate with legacy binaries that
do not sign or authenticate pointers.

The idea is that a dynamic loader or early startup code would issue
this prctl very early after establishing that a process may load legacy
binaries, but before executing any PAC instructions.
---
 .../arm64/pointer-authentication.rst          | 27 +++++++++++++++
 arch/arm64/include/asm/asm_pointer_auth.h     | 19 +++++++++++
 arch/arm64/include/asm/pointer_auth.h         | 10 ++++--
 arch/arm64/include/asm/processor.h            |  5 +++
 arch/arm64/kernel/asm-offsets.c               |  1 +
 arch/arm64/kernel/pointer_auth.c              | 34 +++++++++++++++++++
 include/uapi/linux/prctl.h                    |  3 ++
 kernel/sys.c                                  |  8 +++++
 8 files changed, 105 insertions(+), 2 deletions(-)

diff --git a/Documentation/arm64/pointer-authentication.rst b/Documentation/arm64/pointer-authentication.rst
index 30b2ab06526b..1f7e064deeb3 100644
--- a/Documentation/arm64/pointer-authentication.rst
+++ b/Documentation/arm64/pointer-authentication.rst
@@ -107,3 +107,30 @@ filter out the Pointer Authentication system key registers from
 KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
 register. Any attempt to use the Pointer Authentication instructions will
 result in an UNDEFINED exception being injected into the guest.
+
+
+Enabling and disabling keys
+---------------------------
+
+The prctl PR_PAC_SET_ENABLED_KEYS allows the user program to control which
+PAC keys are enabled in a particular task. It takes two arguments, the
+first being a bitmask of PR_PAC_APIAKEY, PR_PAC_APIBKEY, PR_PAC_APDAKEY
+and PR_PAC_APDBKEY specifying which keys shall be affected by this prctl,
+and the second being a bitmask of the same bits specifying whether the key
+should be enabled or disabled. For example::
+
+  prctl(PR_PAC_SET_ENABLED_KEYS,
+        PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY | PR_PAC_APDBKEY,
+        PR_PAC_APIBKEY, 0, 0);
+
+disables all keys except the IB key.
+
+The main reason why this is useful is to enable a userspace ABI that uses PAC
+instructions to sign and authenticate function pointers and other pointers
+exposed outside of the function, while still allowing binaries conforming to
+the ABI to interoperate with legacy binaries that do not sign or authenticate
+pointers.
+
+The idea is that a dynamic loader or early startup code would issue this
+prctl very early after establishing that a process may load legacy binaries,
+but before executing any PAC instructions.
diff --git a/arch/arm64/include/asm/asm_pointer_auth.h b/arch/arm64/include/asm/asm_pointer_auth.h
index 52dead2a8640..d121fa5fed5f 100644
--- a/arch/arm64/include/asm/asm_pointer_auth.h
+++ b/arch/arm64/include/asm/asm_pointer_auth.h
@@ -31,6 +31,14 @@ alternative_else_nop_endif
 	ldp	\tmp2, \tmp3, [\tmp1, #PTRAUTH_USER_KEY_APDB]
 	msr_s	SYS_APDBKEYLO_EL1, \tmp2
 	msr_s	SYS_APDBKEYHI_EL1, \tmp3
+
+	ldr	\tmp2, [\tsk, #THREAD_SCTLR_ENXX_MASK]
+	cbz	\tmp2, .Laddr_auth_skip_\@
+
+	mrs_s	\tmp3, SYS_SCTLR_EL1
+	bic	\tmp3, \tmp3, \tmp2
+	msr_s	SYS_SCTLR_EL1, \tmp3
+
 .Laddr_auth_skip_\@:
 alternative_if ARM64_HAS_GENERIC_AUTH
 	ldp	\tmp2, \tmp3, [\tmp1, #PTRAUTH_USER_KEY_APGA]
@@ -45,6 +53,17 @@ alternative_else_nop_endif
 	ldp	\tmp2, \tmp3, [\tmp1, #PTRAUTH_KERNEL_KEY_APIA]
 	msr_s	SYS_APIAKEYLO_EL1, \tmp2
 	msr_s	SYS_APIAKEYHI_EL1, \tmp3
+
+	ldr	\tmp2, [\tsk, #THREAD_SCTLR_ENXX_MASK]
+	cbz	\tmp2, .Lset_sctlr_skip_\@
+
+	mrs_s	\tmp1, SYS_SCTLR_EL1
+	mov	\tmp2, #(SCTLR_ELx_ENIA | SCTLR_ELx_ENIB | SCTLR_ELx_ENDA)
+	movk	\tmp2, #SCTLR_ELx_ENDB
+	orr	\tmp1, \tmp1, \tmp2
+	msr_s	SYS_SCTLR_EL1, \tmp1
+
+.Lset_sctlr_skip_\@:
 	.endm
 
 	.macro ptrauth_keys_install_kernel_nosync tsk, tmp1, tmp2, tmp3
diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index c6b4f0603024..d4c375454a36 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -70,14 +70,19 @@ static __always_inline void ptrauth_keys_switch_kernel(struct ptrauth_keys_kerne
 }
 
 extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);
+extern int ptrauth_prctl_set_enabled_keys(struct task_struct *tsk,
+					  unsigned long keys,
+					  unsigned long enabled);
 
 static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
 {
 	return ptrauth_clear_pac(ptr);
 }
 
-#define ptrauth_thread_init_user(tsk)					\
-	ptrauth_keys_init_user(&(tsk)->thread.keys_user)
+#define ptrauth_thread_init_user(tsk) do {				\
+		ptrauth_keys_init_user(&(tsk)->thread.keys_user);	\
+		(tsk)->thread.sctlr_enxx_mask = 0;			\
+	} while (0)
 #define ptrauth_thread_init_kernel(tsk)					\
 	ptrauth_keys_init_kernel(&(tsk)->thread.keys_kernel)
 #define ptrauth_thread_switch_kernel(tsk)				\
@@ -85,6 +90,7 @@ static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
 
 #else /* CONFIG_ARM64_PTR_AUTH */
 #define ptrauth_prctl_reset_keys(tsk, arg)	(-EINVAL)
+#define ptrauth_prctl_set_enabled_keys(tsk, keys, enabled)	(-EINVAL)
 #define ptrauth_strip_insn_pac(lr)	(lr)
 #define ptrauth_thread_init_user(tsk)
 #define ptrauth_thread_init_kernel(tsk)
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 240fe5e5b720..6974d227b01f 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -150,6 +150,7 @@ struct thread_struct {
 #ifdef CONFIG_ARM64_PTR_AUTH
 	struct ptrauth_keys_user	keys_user;
 	struct ptrauth_keys_kernel	keys_kernel;
+	u64				sctlr_enxx_mask;
 #endif
 };
 
@@ -313,6 +314,10 @@ extern void __init minsigstksz_setup(void);
 /* PR_PAC_RESET_KEYS prctl */
 #define PAC_RESET_KEYS(tsk, arg)	ptrauth_prctl_reset_keys(tsk, arg)
 
+/* PR_PAC_SET_ENABLED_KEYS prctl */
+#define PAC_SET_ENABLED_KEYS(tsk, keys, enabled)				\
+	ptrauth_prctl_set_enabled_keys(tsk, keys, enabled)
+
 #ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
 /* PR_{SET,GET}_TAGGED_ADDR_CTRL prctl */
 long set_tagged_addr_ctrl(unsigned long arg);
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 0577e2142284..dac80e16fe35 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -47,6 +47,7 @@ int main(void)
 #ifdef CONFIG_ARM64_PTR_AUTH
   DEFINE(THREAD_KEYS_USER,	offsetof(struct task_struct, thread.keys_user));
   DEFINE(THREAD_KEYS_KERNEL,	offsetof(struct task_struct, thread.keys_kernel));
+  DEFINE(THREAD_SCTLR_ENXX_MASK,offsetof(struct task_struct, thread.sctlr_enxx_mask));
 #endif
   BLANK();
   DEFINE(S_X0,			offsetof(struct pt_regs, regs[0]));
diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
index 1e77736a4f66..8c385b7f324a 100644
--- a/arch/arm64/kernel/pointer_auth.c
+++ b/arch/arm64/kernel/pointer_auth.c
@@ -42,3 +42,37 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
 
 	return 0;
 }
+
+static u64 arg_to_enxx_mask(unsigned long arg)
+{
+	u64 sctlr_enxx_mask = 0;
+	if (arg & PR_PAC_APIAKEY)
+		sctlr_enxx_mask |= SCTLR_ELx_ENIA;
+	if (arg & PR_PAC_APIBKEY)
+		sctlr_enxx_mask |= SCTLR_ELx_ENIB;
+	if (arg & PR_PAC_APDAKEY)
+		sctlr_enxx_mask |= SCTLR_ELx_ENDA;
+	if (arg & PR_PAC_APDBKEY)
+		sctlr_enxx_mask |= SCTLR_ELx_ENDB;
+	return sctlr_enxx_mask;
+}
+
+int ptrauth_prctl_set_enabled_keys(struct task_struct *tsk, unsigned long keys,
+				   unsigned long enabled)
+{
+	u64 sctlr_enxx_mask = tsk->thread.sctlr_enxx_mask;
+	unsigned long addr_key_mask = PR_PAC_APIAKEY | PR_PAC_APIBKEY |
+				      PR_PAC_APDAKEY | PR_PAC_APDBKEY;
+
+	if (!system_supports_address_auth())
+		return -EINVAL;
+
+	if ((keys & ~addr_key_mask) || (enabled & ~keys))
+		return -EINVAL;
+
+	sctlr_enxx_mask |= arg_to_enxx_mask(keys);
+	sctlr_enxx_mask &= ~arg_to_enxx_mask(enabled);
+
+	tsk->thread.sctlr_enxx_mask = sctlr_enxx_mask;
+	return 0;
+}
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..18e1ae4a37a2 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,7 @@ struct prctl_mm_map {
 #define PR_SET_IO_FLUSHER		57
 #define PR_GET_IO_FLUSHER		58
 
+/* Set enabled arm64 pointer authentication keys */
+#define PR_PAC_SET_ENABLED_KEYS		59
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 00a96746e28a..623df216183b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -119,6 +119,9 @@
 #ifndef PAC_RESET_KEYS
 # define PAC_RESET_KEYS(a, b)	(-EINVAL)
 #endif
+#ifndef PAC_SET_ENABLED_KEYS
+# define PAC_SET_ENABLED_KEYS(a, b, c)	(-EINVAL)
+#endif
 #ifndef SET_TAGGED_ADDR_CTRL
 # define SET_TAGGED_ADDR_CTRL(a)	(-EINVAL)
 #endif
@@ -2494,6 +2497,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			return -EINVAL;
 		error = PAC_RESET_KEYS(me, arg2);
 		break;
+	case PR_PAC_SET_ENABLED_KEYS:
+		if (arg4 || arg5)
+			return -EINVAL;
+		error = PAC_SET_ENABLED_KEYS(me, arg2, arg3);
+		break;
 	case PR_SET_TAGGED_ADDR_CTRL:
 		if (arg3 || arg4 || arg5)
 			return -EINVAL;
-- 
2.28.0.163.g6104cc2f0b6-goog

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